cmake: Bump the minimum required Python version to 3.8
cmake: Bump the minimum required Python version to 3.8

Given that 3.6 is now several years old and the current and previous
Ubuntu LTS ship with 3.8 or later, upgrade the minimum required Python
version from 3.6 to 3.8.

Signed-off-by: Carles Cufi <853081dc8dca4fe6a7e3c984752718b5db36e060@nordicsemi.no>

finikorg/zephyr,galak/zephyr,zephyrproject-rtos/zephyr,finikorg/zephyr,zephyrproject-rtos/zephyr,zephyrproject-rtos/zephyr,galak/zephyr,galak/zephyr,finikorg/zephyr,zephyrproject-rtos/zephyr,finikorg/zephyr,finikorg/zephyr,galak/zephyr,galak/zephyr,zephyrproject-rtos/zephyr